This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-275537, filed Sep. 29, 1999, the entire contents of which are incorporated herein by reference.
This invention relates to a color image forming apparatus which forms color images by superimposing single-color images, such as an electrophotographic color electronic copying machine or a color printer.
In many color image forming apparatuses, the following method has been widely used, a specific number of single-color image forming units (normally, four units), defined by subtractive primaries, each unit being composed of a photosensitive member and a developing unit, are arranged in series and a yellow (Y) image, a magenta (M) image, a cyan (C) image, and a black (B) image for reinforcing the contrast between light and shade. Each formed by the corresponding image forming unit are stacked one on top of another in sequence on a paper sheet or a transparent resinous sheet for overhead projectors transported by a transfer belt provided along each image forming unit. Another known method of stacking four images is to transfer four images to an intermediate transfer member and then transfer the stacked images on the intermediate transfer member to a sheet material at one time.
Since such a color image forming apparatus requires four color (four) images to be stacked on top of another accurately, it is often provided with various controls for obtaining exactly aligned images.
For example, in the photosensitive member (drum like) peripheral speed control, the rotation of the drum (photosensitive member) driving motor is controlled to a constant value so that the photosensitive member peripheral speed at which a given point on the outer surface of the photosensitive member provided on each of the four image forming units is moved may become equal to the belt speed at which a given point on the transfer belt rotated by a belt motor is moved. Moreover, in belt peripheral speed control, the rotational speed of the transfer belt motor driving the transfer belt is sensed and the rotation of the belt driving motor is controlled to a constant value so that the photosensitive member peripheral speed may become equal to the belt speed.
Because the contact part where the photosensitive member of each of the four image forming units comes into contact with the transfer belt is separated, such a correction is made as shifts the image forming timing at the stacked part according to the separation.
Actually, however, it is difficult to obtain a stacked image without any shift because of various factors, including a shift in the position when exposure light is projected on each photosensitive member, a shift in the pitch of the photosensitive member (the image forming unit) each other, the slip between the driving roller for driving the transfer belt and the transfer belt, and changes in the peripheral speed of the transfer belt due to fluctuations in the diameter of the driving rollers that drive the transfer belt caused by thermal expansion.
To overcome the disadvantages, there is also provided a control sequence that causes the degree of the color shift in superimposing the images during the power turn-on time or the warm-up time after the cover or the like is opened and closed to restore the operation from the state where a sheet material, i.e., the paper sheet or the resinous sheet got jammed inside the apparatus. There is further provided an image density control sequence for maintaining the image density, the amount of toner adhered, suitably even when the characteristic varies with temperature or time.
Even if various corrections or controls are have been performed as described above, the difference between the sheet material transport speed at which the aligning roller feeding the sheet material to the transfer belt transports the sheet material and the peripheral speed of transfer belt would cause a color shift (a shift in the position of the stacked images).
For example, when the transport speed of the aligning roller is slower than the peripheral speed of the transfer belt, the aligning roller applies a load to the sheet material on the transfer belt in the opposite direction to the direction in which the sheet material is moved (the aligning roller pulls the sheet material), with the result that a color shift occurs throughout the whole surface of the sheet material. Moreover, the effect of vibrations or the like of the paper feed driving system transmitted from the aligning roller causes jitters.
Conversely, when the transport speed of the aligning roller is faster than the peripheral speed of the transfer belt, a large bend occurs in the sheet material in the space defined by an upper guide and a lower guide provided in such a manner that they sandwich the sheet material between them from above and below, in the direction in which the aligning roller transports the sheet material between an electrification roller for causing the sheet material to adhere to the transfer belt by an electrostatic force and the aligning roller. After the bend in the sheet material has grown to the extent that the space cannot absorbs the bend, when the bend stretches, pitching (or waving) takes place in the sheet material in the direction in which the sheet material on the transfer belt is pushed out, shifting the position of the sheet material on the transfer belt, which results in a color shift.
As described above, the sheet transport speed that the aligning roller is required to have has a narrow suitable speed range. It is unfavorable that the sheet transport speed should be too fast or too slow. A single-color-only (black-only) image forming apparatus has no color superimposition. In an image forming apparatus involving digital processing, the density gradation is binarized and expressed by the density of pixels, which makes jitters less liable to take place.
In contrast, a color image forming apparatus requires color superimposition and subjects the density gradation to multivalued processing, forming pixels of different sizes with an equal pitch, which makes jitters conspicuous.
Furthermore, in a small color printer or color copying machine, the size of the apparatus is limited and the distance between the aligning roller and the image forming section cannot be made long. Since the electrification roller for causing paper to adhere to the transfer roller by electrostatic force in the monochrome mode is located in the same position even in the color mode, this reduces the bending space.
As described above, the following problem exists: in the paper sheet bending space defined by sheet guides provided between the aligning roller and transfer belt, an enough space to allow the sheet of paper to bend sufficiently cannot be secured. A known apparatus of this type has the following problem: the length of a sheet of paper that is allowed to bend is 2 mm or less and the proper speed range for the A3 longitudinal size 420 mm is limited to 0.48% or less.
The object of the present invention is to provide a color image forming apparatus which causes the relative difference between the sheet material transport speed at a sheet material feeding unit and that of an image forming unit to fall in a proper speed range, thereby preventing a color shift particularly in a color mode.
The foregoing object is accomplished by providing an image forming apparatus comprising: an image forming section for forming images using developer; a feeding mechanism for feeding an image forming medium to the image forming section; a transport mechanism for transporting the image forming medium fed by the feeding mechanism; a transfer unit for transferring the developer image formed by the image forming section onto the image forming medium; a fixing unit for fixing the transferred developer image; a first sensing unit for recognizing or sensing the transport speed Va of the image forming medium fed from the feeding mechanism to the image forming section; a second sensing unit for recognizing or sensing the transport speed Vb of the image forming medium at the image forming section; and a control unit for comparing the transport speed Va of the image forming medium obtained from the first sensing unit and the transport speed Vb obtained from the second sensing unit and for controlling a proper relative speed with respect to the transport speed Va and the transport speed Vb.
The foregoing object is further accomplished by providing an image forming method in an image forming apparatus including an OHP sheet identify sensor for recognizing or sensing the transport speed Va of a medium fed from an aligning mechanism section to a transfer belt and a registration sensor for recognizing or sensing the transfer speed Vb of the medium on the transfer belt, the image forming method comprising the steps of: comparing the transport speed Va of the medium obtained from the OHP sheet identify sensor with the transport speed Vb of the medium obtained from the registration sensor and adjusting the transport speed Va and the transport speed Vb in such a manner that a state where the transport speed Va is faster than the transport speed Vb by a specific positive ratio of xcex1 % expressed by the following equation is satisfied:
Va=Vbxc3x97(1 +xcex1)
The foregoing object is still further accomplished by providing an image forming apparatus comprising: an image forming section for forming images using developer; a feeding mechanism for feeding an image forming medium to the image forming section; a transport mechanism for transporting the image forming medium fed by the feeding mechanism; a transfer unit for transferring the developer image formed by the image forming apparatus onto the image forming medium; a fixing unit for fixing the transferred developer image; a first sensing unit which is a reflection-type or a transmission-type optical sensor for determining whether the medium is paper or OHP or verifying the passage of the medium and recognizes or senses the transport speed Va of the medium fed from the feeding mechanism to the image forming section; a second sensing unit which is a registration sensor for sensing a shift in the position of an image that is an optical sensor for sensing and measuring various adjustment marks formed on the transfer means and recognizes or senses the transport speed Vb of the medium at the image forming section; and a control unit for comparing the transport speed Va of the medium obtained from the first sensing unit with the transport speed Vb of the medium obtained from the second sensing unit and controlling the transport speed Va of the medium at the feeding mechanism and the transport speed Vb of the medium at the image forming section to specific speeds in such a manner that the following equation is satisfied:
Va=Vbxc3x97(1 +xcex1)
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.